The invention relates to methods for making metallic alloys such as nickel base superalloys into hollow tubes, cylinders, pipes, rings and similar tubular products by melting the alloys in a vacuum or under a low partial pressure of inert gas and subsequently centrifugally casting the melt under vacuum or under a low pressure of inert gas in molds machined from fine grained high density, high strength isotropic graphite revolving around its own axis. The method also relates to a centrifugal casting mold apparatus that includes an isotropic graphite mold.
Nickel base superalloys fabricated in shapes such as seamless rings, hollow tubes and pipes find many engineering applications in jet engines, oil and chemical industries and other high performance components. Complex highly alloyed nickel base superalloys are produced in seamless ring configurations for demanding applications in jet engines such as turbine casings, seals and rings. FIG. 1 shows a diagram of turbine casing 10 and a compressor casing 20. The turbine casing 10 is made of high temperature nickel base superalloys. Attached FIG. 2 also shows a diagram of a turbine casing 30 made of high temperature nickel base superalloys. Seamless rings can be flat (like a washer), or they can feature higher vertical walls (approximating a hollow cylindrical section). Heights of rolled rings range from less than an inch up to more than 9 ft. Depending on the equipment utilized, wall-thickness/height ratios of rings typically range from 1:16 up to 16:1, although greater proportions have been achieved with special processing.
The two primary processes for forging rings differ not only in equipment, but also in quantities produced. Also called ring forging, saddle-mandrel forging on a press is particularly applicable to heavy cross-sections and small quantities. Essentially, an upset and punched ring blank is positioned over a mandrel, supported at its ends by saddles. As the ring is rotated between each stroke, the press ram or upper die deforms the metal ring against the expanding mandrel, reducing the wall thickness and increasing the ring diameter.
In continuous ring rolling, seamless rings are produced by reducing the thickness of a pierced blank between a driven roll and an idling roll in specially designed equipment. Additional rolls (radial and axial) control the height and impart special contours to the cross-section. Ring rollers are well suited for, but not limited to, production of larger quantities, as well as contoured rings. In practice, ring rollers produce seamless rolled rings to closer tolerances or closer to finish dimensions. FIGS. 3A-3G show schematically the various steps of seamless rolled ring forging process operations. FIG. 4 shows a ring rolling machine in operation.
FIGS. 3A-3G show an embodiment of a seamless rolled ring forging process operation to make a ring 40. FIG. 3A shows the ring rolling process typically begins with upsetting of the starting stock 42 on flat dies 44 at its plastic deformation temperaturexe2x80x94in the case of grade 1020 steel, approximately 2200 degrees Fahrenheit to make a relatively flatter stock 43. FIG. 3B shows that piercing the relatively flatter stock 43 involves forcing a punch 45 into the hot upset stock causing metal to be displaced radially, as shown by the illustration. FIG. 3C shows a subsequent operation, namely shearing with a shear punch 46, serves to remove a small punchout 43A to produce an annular stock 47. FIG. 3D shows that removing the small punchout 43A produces a completed hole through the annular stock 47, which is now ready for the ring rolling operation itself. At this point the annular stock 47 is called a preform 47. FIG. 3E shows the doughnut-shaped preform 47 is slipped over the ID (inner diameter) roll 48 shown from an xe2x80x9cabovexe2x80x9d view. FIG. 3F shows a side view of the ring mill and preform 47 workpiece, which squeezes it against the OD (outer diameter) roll 49 that imparts rotary action. FIG. 3G shows that this rotary action results in a thinning of the section and corresponding increase in the diameter of the ring 40. Once off the ring mill, the ring 40 is then ready for secondary operations such as close tolerance sizing, parting, heat treatment and test/inspection.
FIG. 4 shows a photograph of a ring 40 roll forging machine in operation.
Even though basic shapes with rectangular cross-sections are common, rings featuring complex, functional cross-sections are produced by machining or forging from simple rings to meet virtually any design requirements. Aptly named, these xe2x80x9ccontouredxe2x80x9d rolled rings can be produced in many different shapes with contours on the inside and/or outside diameters.
Production of superalloy rings from forging billets requires multiple steps by ring rolling. These alloys are difficult to hot work and can be hot deformed with small percentage of deformation in each step of ring roll forging. After each deformation operation, the outside and inside diameters of the stretched ring need to be ground to remove oxidized layers and forging cracks before reheating the ring for the next cycle of hot forging. Because of the extensive fabrication steps involved, the production costs are very high and yields are low. Typically, a 60 inch diameter ring weighing 250 lbs. suitable for application as a large jet engine casing is produce by ring roll forging of a starting billet weighing 2000 lbs. The high loss of expensive materials during fabrication steps results in high cost of the finished products.
The conventional route of tube making typically includes argon-oxygen decarburization (AOD) melting, continuous casting, hot rolling, boring, and extrusion. This route is mainly used for the high volume production of tubes up to 250 mm diameter. However, complex nickel base superalloys that are prone to macrosegregation are difficult or impossible to hot work.
Centrifugal casting complements the conventional tube making process and also offers considerable flexibility in terms of tube diameter and wall thickness. The mechanical properties of centrifugally cast tubes are often equivalent to conventionally cast and hot-worked material. The uniformity and density of centrifugal castings approaches that of wrought material, with the added advantage that the mechanical properties are nearly equal in all directions. Although many engineering ferrous and non-ferrous alloys which are amenable to processing by air melting and casting can be conveniently processed in tubes by centrifugal casting in air. However, complex nickel base superalloys require melting and casting in vacuum. Furthermore, during high speed rotation of the centrifugal mold lined with high purity ceramics, the highly reactive nickel base superalloy melts are likely to cause cracking and spalling of the ceramic liner leading to formation of very rough, outside surface of the cast tube. The ceramic liners spalling off the mold are likely to get trapped inside the solidified superalloy tube as detrimental inclusions that will significantly lower fracture toughness properties of the finished products.
There is a need for an improved cost effective process for making highly alloyed complex such as nickel based superalloys as tubes and seamless rings with simple or contoured cross sections which can be inexpensively machined into final shapes suitable for jet engine and other high performance engineering applications.
The term superalloy is used in this application in conventional sense and describes the class of alloys developed for use in high temperature environments and typically having a yield strength in excess of 100 ksi at 1000 degrees F. Nickel base superalloys are widely used in gas turbine engines and have evolved greatly over the last 50 years. As used herein the term superalloy will mean a nickel base superalloy containing a substantial amount of the xcex3xe2x80x2 (Ni3Al) strengthening phase, preferably from about 30 to about 50 volume percent of the xcex3xe2x80x2 (gamma prime) phase. Representative of such class of alloys include the nickel base superalloys, many of which contain aluminum in an amount of at least about 5 weight % as well as one or more of other alloying elements, such as titanium, chromium, tungsten, tantalum, etc. and which are strengthened by solution heat treatment. Such nickel base superalloys are described in U.S. Pat. No. 4,209,348 to Duhl et al. and U.S. Pat. No. 4,719,080. Other nickel base superalloys are known to those skilled in the art and are described in the book entitled xe2x80x9cSuperalloys IIxe2x80x9d Sims et al., published by John Wiley and Sons, 1987.
Other references incorporated herein by reference in their entirety and related to superalloys and their processing are cited below:
xe2x80x9cInvestment-cast superalloys challenge wrought materialsxe2x80x9d from Advanced Materials and Process, No. 4, pp. 107-108 (1990).
xe2x80x9cSolidification Processingxe2x80x9d, editors B. J. Clark and M. Gardner, pp. 154-157 and 172-174, McGraw-Hill (1974).
xe2x80x9cPhase Transformations in Metals and Alloysxe2x80x9d, D. A. Porter, p. 234, Van Nostrand Reinhold (1981).
Nazmy et al., The effect of advanced fine grain casting technology on the static and cyclic properties of IN713LC. Conf: High temperature materials for power engineering 1990, pp. 1397-1404, Kluwer Academic Publishers (1990).
Bouse and Behrendt, Mechanical properties of Microcast-X alloy 718 fine grain investment castings, Conf: Superalloy 718: Metallurgy and applications, Publ:TMS pp. 319-328 (1989).
Abstract of U.S.S.R. Inventor""s Certificate 1306641 Published Apr. 30, 1987.
WPI Accession No. 85-090592/85 and Abstract of JP 60-40644 (KAWASAKI) Published Mar. 4, 1985.
WPI Accession No. 81-06485D/81 and Abstract of JP 55-149747 (SOGO) Published Nov. 21, 1980.
Fang, J: Yu, B Conference: High Temperature Alloys for Gas Turbines, 1982, Liege, Belgium, Oct. 4-6, 1982, pp. 987-997, Publ: D. Reidel Publishing Co., P.O. Box 17, 3300 AA Dordrecht, The Netherlands (1982).
Processing techniques for superalloys have also evolved as evident from the following references incorporated herein by reference in their entirety, and many of the newer processes are quite costly.
U.S. Pat. No. 3,519,503 describes an isothermal forging process for producing complex superalloy shapes. This process is currently widely used, and as currently practiced requires that the starting material be produced by powder metallurgy techniques. The reliance on powder metallurgy techniques makes this process expensive.
U.S. Pat. No. 4,574,015 deals with a method for improving the forgeability of superalloys by producing overaged microstructures in such alloys. The gamma prime phase particle size is greatly increased over that which would normally be observed.
U.S. Pat. No. 4,579,602 deals with a superalloy forging sequence which involves an overage heat treatment.
U.S. Pat. No. 4,769,087 describes another forging sequence for superalloys.
U.S. Pat. No. 4,612,062 describes a forging sequence for producing a fine grained article from a nickel base superalloy.
U.S. Pat. No. 4,453,985 describes an isothermal forging process which produces a fine grain product.
U.S. Pat. No. 2,977,222 incorporated herein by reference describes a class of superalloys similar to those to which the invention process has particular applicability.
It is well known to make a metal shape by a centrifugal casting process in which molten metal is poured into a hollow mould which is rotating. Centrifugal casting provides the advantage of achieving segregation of impurities towards the axis of rotation and away from the external surface of the casting since impurities generally encountered are of lower density than the metal of the casting. Moreover, centrifugal casting enables the production of hollow cast shapes of controlled wall thickness without the need for central cores although, if desired, the rotating mould can be filled sufficiently so as to provide a shape without a central cavity. In either case the part of the casting containing the impurities can be removed, for example by machining.
Hitherto such centrifugal casting has been used with permanent moulds for metal shapes of relatively simple external surface configuration such as generally cylindrical. By providing a sand mould of appropriate shape within a container, generally made of steel, the external surface of the casting may be provided with a more complex configuration, within constraints imposed by the difficulty, complexity and expense of removing rigid patterns, typically of wood, for producing the sand mould, even when the rigid patterns are made collapsible to facilitate removal.
There is a demand for metal shapes, particularly hollow shapes such as gas turbine engine casings, having an external shape of relatively high complexity and precision than it has hitherto been possible, or economically possible, to manufacture by centrifugal casting.
U.S. Pat. No. 6,116,327 to Beighton incorporated herein by reference discloses a method of making a metal shape comprising the steps of supplying molten metal into a ceramic shell mould mounted in a container, spinning the container and the shell mould therein about an axis and permitting the metal to solidify in the shell mould and thereafter removing, for example by breaking, the shell mould to expose the metal shape. The ceramic shell moulds made by providing a pattern of flexible elastically deformable material of a required shape and supported on a mandrel, applying at least one coating of hardenable refractory material to said pattern to form a rigid shell and removing the mandrel from supporting relationship with the pattern and subsequently removing the pattern from the shell by elastically deforming the pattern. The pattern is made by molding the material in a master mold of a required shape and removing the pattern from the master mold, after the pattern has set, by elastically deforming the pattern.
U.S. Pat. No. 5,826,322 Hugo, et al. incorporated herein by reference discloses the production of particles from castings (10) of metals from the group of the lanthanides, aluminum, boron, chromium, iron, calcium, magnesium, manganese, nickel, niobium, cobalt, titanium, vanadium, zirconium, and their alloys, which have solidified in an oriented manner, especially for the production of materials from the group of magnetic materials, hydrogen storage elements (hydride storage elements), and battery electrodes, a melt of the metal is applied in a nonreactive atmosphere to the inside of an at least essentially cylindrical cooling surface (9) according to the principle of centrifugal casting. The cylinder rotates at high speed around a rotational axis, and the melt is cooled proceeding from the outside toward the inside with an essentially radial direction of solidification. The hollow casting (10) is then reduced to particles. The melt is preferably applied to the rotating cooling surface (9) in a thickness which is no more than 10%, and preferably no more than 5%, of the diameter of the cooling surface (9), and the diameter of the cooling surface (9) is at least 200 mm, and preferably at least 500 mm.
The use of graphite in investment molds has been described in U.S. Pat. Nos. 3,241,200; 3,243,733; 3,265,574; 3,266,106; 3,296,666 and 3,321,005 all to Lirones and all incorporated herein by reference. U.S. Pat. Nos. 3,257,692 to Operhall; 3,485,288 to Zusman et al.; and 3,389,743 to Morozov et al. disclose carbonaceous mold surface utilizing graphite powders and finely divided inorganic powders termed xe2x80x9cstuccosxe2x80x9d and are incorporated herein by reference.
U.S. Pat. No. 4,627,945 to Winkelbauer et al., incorporated herein by reference, discloses injection molding refractory shroud tubes made from alumina and from 1 to 30 weight percent calcined fluidized bed coke, as well as other ingredients. The ""945 patent also discloses that it is known to make isostatically-pressed refractory shroud tubes from a mixture of alumina and from 15 to 30 weight percent flake graphite, as well as other ingredients.
It is an object of the invention to centrifugally cast nickel base superalloys as tubes, pipes and rings under vacuum or partial pressure of inert gas in isotropic graphite molds rotating around its own axis.
It is another object of the present invention to provide a centrifugal casting apparatus which includes an isotropic graphite mold.
This invention relates to a process for making various metallic alloys such as nickel based superalloys as engineering components such as rings, tubular parts and pipes by vacuum induction melting of the alloys and subsequent centrifugal casting of the melt in graphite molds rotating around its own axis under vacuum. More particularly, this invention relates to the use of high density, high strength isotropic graphite. FIG. 5 shows a schematic drawing of the centrifugal vacuum casting equipment for casting nickel base superalloys in a rotating isotropic graphite mold under vacuum to make a hollow tube casting in accordance with the scope of the present invention.
From a vessel in a vacuum chamber, molten metal is poured through a launder into a rotating isotropic graphite mold. With centrifugal casting, the rotating isotropic graphite metal mold revolves under vacuum at high speeds in a horizontal, vertical or inclined position as the molten metal is being poured. The axis of rotation may be horizontal or inclined at any angle up to the vertical position. Molten metal, poured into the spinning mold cavity, is held against the wall of the mold by centrifugal force. The speed of rotation and metal pouring rate vary with the alloy and size and shape being cast.
As the molten metal alloy is poured into the rotating isotropic graphite mold, it is accelerated to mold speed. Centrifugal force causes the metal to spread over and cover the mold surface. Continued pouring of the molten metal increases the thickness to the intended cast dimensions. Rotational speeds vary but sometimes reach more than 150 times the force of gravity on the outside surface of the castings.
Once the metal is distributed over the mold surface, solidification begins immediately. Metal feeds the solid-liquid interface as it progresses toward the bore. This, combined with the centrifugal pressure being applied, results in a sound, dense structure across the wall with impurities generally being confined near the inside surface. The inside layer of the solidified part can be removed by boring if an internal machined surface is required. Accordingly, the hollow tube casting is solidified and recovered.
For specialized engineered shapes, centrifugal casting offers the following distinct benefits of nickel base superalloys:
any superalloy common to static pouring under vacuum can be centrifugally cast in accordance with the present invention as a tubular product, ring and pipe; and
mechanical properties of centrifugally cast nickel base superalloys according to the present invention will be excellent.
Centrifugal castings of nickel base superalloy can be made in almost any required length, thickness and diameter. Because the mold forms only the outside surface and length, castings of many different wall thicknesses can be produced from the same size mold. The centrifugal force of this process keeps the casting hollow, eliminating the need for cores.
Horizontal centrifugal casting technique is suitable for the production of superalloy pipe and tubing of long lengths. The length and outside diameter are fixed by the mold cavity dimensions while the inside diameter is determined by the amount of molten metal poured into the mold.
Castings other than cylinders and tubes also can be produced in vertical casting machines. Castings such as controllable pitch propeller hubs, for example, can be made using this variation of the centrifugal casting process.
The outside surface of the casting or the mold surface proper can be modified from the true circular shape by the introduction of flanges or small bosses, but they must be generally symmetrical about the axis to maintain balance. The inside surface of a true centrifugal casting is always cylindrical. In semi-centrifugal casting, a central core is used to allow for shapes other than a true cylinder to be produced on the inside surface of the casting.
The uniformity and density of centrifugal castings approaches that of wrought material, with the added advantage that the mechanical properties are nearly equal in all directions. Most alloys can be cast successfully by the centrifugal process, once the fundamentals have been mastered. Since no gates and risers are used, the yield or ratio of casting weight-to-weight of metal is high.
High tangential strength and ductility will make centrifugally cast nickel base superalloys well-suited for torque- and pressure-resistant components, such as gears, engine bearings for aircraft, wheel bearings, couplings, rotor spacers, sealed discs and cases, flanges, pressure vessels and valve bodies.
Superalloy melts do not react with high density, ultra fine grained isotropic graphite molds and hence, the molds can be used repeatedly many times thereby reducing significantly the cost of fabrication of centrifugally cast superalloy components compared to traditional process. Near net shape parts can be cast, eliminating subsequent operating steps such as machining.